The conversion of a liquid or thermoplastic uncured epoxy to a tough thermoset solid epoxy may be brought about by the addition of a wide range of curing agents (also known as hardeners, activators or catalysts). These curing agents differ widely in their effect upon the uncured resin, for example, the curing action may be exothermic with some curing agents while requiring the application of external heat with others. Furthermore, the curing epoxy group may react anionically or cationically. Since there are over 50 different epoxy resins available, and hundreds of known curing agents, the epoxy formulator has available to him a very wide diversity of resin curing agent combinations.
While basic curing agents such as Lewis bases, inorganic bases, primary and secondary amines and amides are most commonly used curing agents, acid curing agents are preferred in many formulations. Of these acid curing agents, carboxylic acid, anhydrides, dibasic organic acids, phenols and Lewis acids are types of curing agents which can be made to successfully bring about the curing reaction. By the term "Lewis acids" is meant those compounds containing empty orbital positions in the outer shell of one of the atoms and because of this, there acids are attracted to areas of increased electron density.
Of the Lewis acids, boron trifluoride and amine complexes thereof are useful curing agents (see Lee & Neville, Handbook of Epoxy Resins, McGraw Hill 1967 at pp. 5-13 through 5-16). These catalysts, while effective curing agents, have the disadvantage of being hard to control and often lead to rapid reaction rates with an undesirably high exotherm which can cause thermal degradation of the polymer with its concomitant darkening or gas evolution.
Several approaches have been proposed to help control the rapid exotherm. One such approach is disclosed in U.S. Pat. No. 3,117,099 which suggests a mixture of boron trifluoride and stannous compounds of the general formula: Sn(OR).sub.2 in which R is a monovalent hydrocarbon radical, saturated or unsaturated, branched chain or straight chain, containing 1 to 18 carbon atoms, preferably 3 to 12. The use of this curing system is, however, limited to epoxide compositions whih contain either a cyclohexene oxide or a cyclopentene group and, thus, is not available for the wide variety of epoxides which do not contain these groups.
Another patent disclosing boron trifluoride (BF.sub.3) addition products is U.S. Pat. No. 2,824,083 which suggests products of BF.sub.3 and an amine, amide, phenol or ether dissolved in a liquid polyol and preferably a liquid polyalkylene glycol having a molecular weight above 100. These curing systems are limited in use, however, because most require that the formulation be heated to a high temperature, e.g. 212.degree. F to 392.degree. F, to give a good rate of cure. Another patent disclosing amine-boron trifluoride complexes is U.S. Pat. No. 2,717,885. The curing agents there described remain unreactive at room temperature and must be heated to an elevated temperature to cure (most examples of this patent used 392.degree. F with the lowest curing temperature used being 266.degree. F).
U.S. Pat. No. 3,432,440 suggests various fluoborate salts in combination with an acid or hydrolyzable ester thereof. Examples suggested in this patent include Zn(BF.sub.4).sub.2, Ni(BF.sub.4).sub.2, Cu(BF.sub.4).sub.2, Sn(BF.sub.4).sub.2. Zn(BF.sub.4).sub.2 and Ni(BF.sub.4).sub.2. The patent states that activation of the fluoborate salt by a relatively strong acid is essential and that the release of HBF.sub.4 may be delayed by providing a hydrolyzable ester of the strong acid rather than the strong acid itself. Examples of such hydrolyzable esters include triphenyl, phosphite, diphenyl phosphite, dimethyl hydrogen phosphite, n-butyl acid phosphate, diethyl oxalate, triethyl phosphite and dimethyl sulfate.
U.S. Pat. No. 2,970,983 includes the use of BF.sub.3 complexes as a suggested curing agent. Such complexes are not listed among the "preferred curing agents" of this patent.
Many polyepoxides exhibit an excellent ability to bond to a wide range of substrates, however, a still further improvement is often desired. U.S. Pat. No. 3,014,893 discloses that the addition of an inorganic amphoteric oxide to an epoxy resin will significantly improve the adhesive bond strength of the cured epoxy as compared to a conventionally cured epoxy lacking the inorganic amphoteric oxide. Oxides of phosphorous, arsenic, antimony, bismuth, tin, lead, and germanium are suggested.
The amphoteric oxides may be used alone or in conjunction with a conventional organic amine or organic anhydride curing agent. In order to effect cure, heating is necessary and temperatures of 500 to 600.degree. F are recommended.
Although epoxy resins are not exceptionally flammable, they, like most plastics, when exposed to flame will ignite and burn. Various means are known to decrease the flammability of burning rate of epoxy resins. Novolac based epoxy resins may be based on chlorinated phenols to impart flame resistance to the cured compositions. Chlorine in various forms is used in other epoxy resins to impart flame resistance but often has a tendency to lower thermal stability particularly in the case of amine cures since heat aging may cause the evolution of hydrochloric acid. Brominated resins are also used where flame resistance is needed but such resins are relatively unstable above 392.degree. F.
For some applications decreased density, decreased cost or increased insulation properties is needed and thus the foaming of epoxy resins is suggested. Epoxy foams are characterized by good adhesive strengths, low water absorption, good dimensional stability, good heat resistance and generally good resistance to chemical attack. Such foams have found use as lightweight potting compounds for electronic equipment and for laod bearing sections of honeycomb members. They have also found use as thermal insulators. Two basic types of epoxy foam formulations are in wide use, namely, chemical foams and syntactic foams. Chemical foams are produced by the release of a gas through a chemical reaction or vaporization whereas syntactic foams are obtained by the incorporation of a prefoamed or low density filler. Epoxy foams have not found a broad based acceptance, however, due in part to the high cost of forming most foams.
Because of its low cost and excellent combination of properties, epoxy resins based upon epichlorohydrin and bisphenol-A are preferred for a wide variety of applications. A curing system which will be effective to give a controlled desirable amount of exotherm without the necessity of the addition of external heat to initiate reaction and which is effective for a wide variety of epoxy resins including those made from epichlorohydrin and bisphenol-A is needed. The utilization of such a curing system to help give specialized properties such as improved adhesion, flame retardancy or foamability is highly desirable.